Tests of a vaccine which, it was hoped, might reverse the progress of Alzheimer's disease, have been stopped after patients fell ill.

The news is a blow to those who are hoping for a treatment which can do more than slow down the progress of the incurable disease.

The vaccine was being tested in four European countries and 11 hospitals in the US.

The trials were designed to test the safety of the formula before moving on to see how effective it was at halting Alzheimer's.

However, 12 people out of the 360 volunteers - all with mild or moderate cases of the disease - became seriously ill with inflammation of the brain.

They had symptoms which ranged from fever, headache and vomiting, to muscle weakness and seizures.

All are being treated and are "responding appropriately", said the pharmaceutical company Elan, which had been developing the vaccine.

'Puzzling'

The idea of the vaccine was to prime the body's own immune system to attack and destroy a body chemical called beta amyloid, the ingredient of "plaques" which appear in great numbers in the brains of Alzheimer's patients.

It is thought these plaques trigger the death of brain cells, and the subsquent progressive loss of cognitive function.

One theory about what has happened in the 12 cases is an extreme immune reaction causing inflammation, in the same way that some vaccines cause the injection site to swell up.

In this case, as the target of the vaccination was the brain, the inflammation occurred there - which can be far more dangerous than a swollen arm.

Dr Marilyn Albert, the head of the Amercian Alzheimer's Association committee, said: "I think everyone is puzzled. They can't go forward with this formulation."

At present, there is no Alzheimer's treatment which can do more than relieve the symptoms of the condition in some patients with mild or moderate forms of the disease.